They're invading by the millions — swarms and swarms of them, their black and orange masts fluttering against the spring air from Cape May to Sussex in one of the greatest butterfly migration in decades.

The Red Admiral, ubiquitous but rarely noticed by the layman, is everywhere this year.

"This is a giant invasion. It’s really pretty spectacular," says Jeffrey Glassberg, president of the North American Butterfly Association, headquartered near Morristown. "We’re to the point where non-butterflying people take notice."

Spend any time outside this month and you’ll probably see a Red Admiral, known for its orange splotches on each wing. Even in New York’s Central Park, in the middle of a metropolis, the butterflies are dotting every plant. Down the shore, especially close to the water, there have been reports of hundreds — and even thousands — flying north together. There are, of course, many of these butterflies in other northern states.

Robert Sciarrino/The Star-LedgerDr. Jeffrey Glassberg, president of the North American Butterfly Association and author of many books on the subject, says the red admiral butterfly is invading New Jersey in far greater numbers than usual, and they're doing so much earlier in the year.

"There are huge numbers of them in western New York and Michigan and Minnesota," says Glassberg, author of many books on the subject.

What's happening?

Red Admirals can’t survive in the cold. Each fall they flee south, spending the winter in the Carolinas, Georgia and other deep-south states. Like other butterfly species, they lay eggs, die and more are born. When spring comes, new generations begin migrating, repopulating northern states and even reaching Canada. Usually, it’s a migration that goes unnoticed by most people.

Every 10 years or so, however, the Red Admiral population booms and spring brings a great happening, says Pat Sutton, a naturalist and writer from Cape May. The last major migration was 2001, so we were due, she says.

But this migration is tremendous, she says. It’s likely spurred by the mild winter, which would have allowed Red Admirals to survive farther north and multiply in great number.

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"This year is probably moreso than the other big flights we’ve had in 1981, 1990 and 2001," says Sutton, who used to work for the New Jersey Audubon’s Cape May Bird Observatory and now works as a freelance educator.

In 1990, for example, there were so many Red Admirals that the ones who died over the ocean — perhaps pushed out by winds — actually washed ashore in noticeable numbers, Sutton says. This year’s migration could be even larger than that.

"This seems to be one of the biggest I’ve ever witnessed," she says. "It’s a fascinating happening."

Jane Scott, a Florham Park resident and member of the North American Butterfly Association, spotted a great number of the Red Admirals and, perhaps, some similar butterflies in Red Bank last week.

"Unbelievable numbers of butterflies. Just constant. It wasn’t just like one cloud of them, but rather just continuously coming by in twos and threes," Scott says. "It was incredible."

What’s so great about this migration, butterfly watchers say, is that everyone gets to see the little creatures — not just those who are on the lookout. It’s a science lesson for the willing and unwilling.

"The natural world, I think, is something that gives us all hope," says Sutton, who hopes the migration will inspire some to grow gardens to attract wildlife. "Here is a wonderful natural history happening that is exciting — life giving."